This application is the national phase of international application PCT/IT99/00284 filed Sep. 6, 1999 which designated the U.S.
The present invention relates to a sol-gel process for the preparation of thick glassy films of silicon oxide or based on silicon oxide and to the thick films thus obtained.
In the technology of solid state, with the term xe2x80x9cfilmxe2x80x9d is meant a thin layer of a material having a thickness generally comprised between a few tens of nanometers (nm) and a few tens of micrometers (xcexcm), said layer being supported over a substrate of another material, generally a flat geometry.
The term xe2x80x9cthickxe2x80x9d typically refers to films of thickness larger than 1 xcexcm.
Thick glossy films, deposited over a suitable substrate, are the object of extensive research in view of their foreseen use in the field of telecommunications, particularly telecommunications on optical and electro-optical cables.
In the past, telephone communications and data transmissions were realised transforming the signal into electronic impulses that were transmitted by means of cables of an electrically conductive material, generally copper.
Nowadays, in particular for the long distances, transmissions on electrical cables have been almost completely replaced by transmissions an optical fibers. As known, the optical fibers are glassy fibers whose structure comprises at least a central part, called nucleus, and an outer part, called mantle, made of glasses having slightly different chemical compositions; the different chemical composition gives rise to a difference in the refractive index of the two materials that allows confining the optical signal in the nucleus. Commonly the mantle is made of pure silicon oxide, whilst the nucleus is made of a mixed oxide based on silicon oxide containing from a few percent to about 10% by mole of different oxides such as germanium oxide.
The optical fibers offer several advantages over electrical cables as means for information transmission, such as a lower level of noise and lower signal attenuation as well as a higher amount of information transmitted per unit time, resulting in a higher transmission rate.
Despite these advantages, it has not been possible so far to fully exploit the potentiality of optical communications: in fact, a complete communication system requires devices for processing signals, for instance for transforming voice into signal at the two ends of the cable in telephonic transmissions, or for amplifying the signal along the fibre, that is rendered necessary due to unavoidable attenuation of the same signal. More generally, the so-called operation of signal commutation that is needed for delivering the same signal in the network requires suitable devices.
To this end, traditional electrical devices (electronic switches) are presently used, and generally any operation on the signal requires a conversion into electrical signal followed by a possible further conversion back to optical signal. In these operations time and signal quality are lost. As a consequences, a strong need is felt for optical or electro-optical devices capable of guiding an optical signal as well as of performing on it commuting operations comparable to those operated by electronic devices on electrical signals.
The main features that optical devices must have, are:
material of very high transmittance, requiring absence of inclusions and mechanical defects;
possibility of controlling through chemical composition the refractive index, that must be at least a few percent units higher than that of surrounding materials;
flat geometry, for easy fit into automated production lines;
thickness of a few xcexcm, preferably about 2 and 20 xcexcm.
In order to ease integration of these devices into production and communication lines, the substrate should preferably be made of silicon or silicon oxide.
Such devices are presently produced according to physical techniques, among which thermal oxidation of silicon, and those known as Sputtering, Chemical Vapor Deposition and Flame Hydrolysis can be cited. Another method consists in the vacuum deposition on a silicon substrate of microparticles of silicon oxide obtained according to the Flame Hydrolysis technique.
However, these productions are complex, requiring costly working chambers and tools; some of these, such as silicon thermal oxidation, have a limit in the film thickness that can be obtained, while others are exceedingly slow and are often characterised by low productivity and too high costs, so as not to allow an actual industrial exploitation of optical devices.
The most economically promising technology for massive production of glassy films on substrates is sol-gel. Under the name sol-gel are gathered different procedures for the preparation of oxides of one or more elements in form of porous bodies, ceramics or glasses.
While differing from each other in the specific details, all sol-gel procedures share the following phases:
preparation of a xe2x80x9csolxe2x80x9d, a solution or suspension in water, alcohol or hydroalcoholic mixtures of precursors of the elements whose oxides is to be prepared. Generally used as precursors are the alkoxides, of formula M(OR)n, where M represents the element whose oxide is desired, the group xe2x80x94OR is the alkoxide moiety, and n represents the valence of element M; soluble salts of the element M, such as chlorides; nitrates and exceptionally oxides, may be used in place of alkoxides. During this phase the precursors begin to hydrolyse, that is, alkoxide moieties or other anions bonded to element M are replaced by xe2x80x94OH groups;
sol gelation, requiring from a few seconds up to some days, depending on chemical composition and temperature of the solution; during this phase hydrolysis of the possibly remaining precursor is completed and condensation occurs, consisting in the reaction of xe2x80x94OH groups belonging to different molecules with formation of one free water molecule and an oxygen bridge between atoms M, Mxe2x80x2 (alike or different), according to the reaction:
(HO)nxe2x88x921Mxe2x80x94OH+HOxe2x80x94Mxe2x80x2(OH)mxe2x88x921xe2x86x92(HO)nMxe2x80x94Oxe2x80x94Mxe2x80x2(OH)m+H2O xe2x80x83xe2x80x83(I) 
The product obtained in this phase is called alcogel, hydrogel depending on the cases, or more generally xe2x80x9cgelxe2x80x9d as widely used in the English literature.
gel drying; in this phase the solvent is removed by simple evaporation or through hypercritical transformation into gas inside an autoclave; there is obtained an extremely porous dry body, that may have an apparent density ranging from about 10% to about 50% of the theoretical density of the oxide of that composition;
dry gel densification by thermal treating at a temperature generally comprised between 800xc2x0 C. and 1200xc2x0 C. depending on the gel chemical composition and on the parameters of the previous process phases; in this phase the porous gel densifies obtaining a glassy or ceramic compact oxide of theoretical density, with a linear shrinkage of about 50%.
If gelation phase is not too fast, it is possible to lay a liquid film of sol on a substrate, eventually resulting in a oxide supported film. Obtaining a oxide film on a substrate in this way is however easily feasible only for a thickness up to some tenths of micrometer. Up to such values of thickness, cohesive forces in the film are weak, and forces adhering the film on the substrate prevail, so that during the densification phase there is not in-plane shrinkage of the film and densification only involves its thickness decrease. At values of thickness above one micrometer, on the other hand, inner cohesive forces of the film become prevailing and during densification in-plane shrinking of the film takes place as well: the result is film fragmentation into xe2x80x9cislandsxe2x80x9d spread over the substrate surface and poor adhesion of the film to the substrate.
This thickness of about 1 xcexcm represents a technological limit for sol-gel technique, as indicated for instance in xe2x80x9cSOL-GEL science: the physics and chemistry of SOL-GEL processingxe2x80x9d, Brinker and Scherer, Academic Press, 1990, a comprehensive review of the knowledge in the field. As already stated above, films prepared in this way are defined thin or thick when they have a thickness below or above about 1 xcexcm, respectively.
For the production of thick films through the sol-gel technique it has been proposed to prepare a sol containing, in addition to normal precursors, a dense material in the form of nanospheres, that is, spheres of dimensions of about 10 nm. This approach is exposed in the paper xe2x80x9cSOL-GEL derived thick coatings and their thermomechanical and optical propertiesxe2x80x9d, Menning et al., SPIE Vol. 1758, SOL-GEL Optics II (1992), pages 125-134. This technique however can hardly be implemented practically; besides, despite the fact that the first papers on the technique were published more then five years ago, actual feasibility of thick films by this route has not been proven yet.
Another proposed approach is to prepare thick films through repeated depositions of thin films; any single layer must be densified before deposition of the subsequent layer. An example of this kind of procedure is given in xe2x80x9cDeposition of thick silica-titania SOL-GEL films on Si substratesxe2x80x9d Syms et al., Journal of Non-Crystalline Solids, 170 (1994), pages 223-233. According to the literature, by this way it is possible to prepare multilayer thick films. On the other hand, as stated in the cited paper, in order to obtain films of good mechanical and optical characteristics any single layer must have a thickness not higher than about 0.25 xcexcm, so that production of a films of thickness about 10 xcexcm requires about 40 deposition and densification steps.
Thus, the production of large amounts of flat waveguides by the sol-gel route is still an open problem.
It is thus an object of the present invention to provide a sol-gel process for the preparation of thick glassy films of silicon oxide or based on silicon oxide, as well as to provide glassy supported films of thickness higher than 1 xcexcm, preferably between 2 and 20 xcexcm.
According to the present invention, these objects are obtained with a sol-gel process for the preparation of thick glassy (vitreous) film of silicon oxide or mixed oxides containing silicon oxide, comprising preparing a sol from a solution or a suspension of precursor elements in water, alcohol or hydroalcoholic mixtures, said precursor elements comprising silicon and, optionally, one or more elements selected from the group consisting of germanium, aluminum, titanium, and zirconium. The molar ratio of the precursor elements of silicon and the sum of the optional precursor elements is greater than or equal to 1:1, said sol comprising a water solution and an acid containing at least 10 moles of H2O per each mole of said precursor elements and having a pH ranging between 0.3 and 1.5 to form a sol. Hydrolysis of the precursor elements is undertaken, after which about 0.7 to about 3.0 moles of SiO2 per mole of the precursor elements is added to the sol. A film of the sol is formed on a substrate, and the sol film is gelled via solvent evaporation. The resulting gel film is subjected to densification through thermal treatment to form a vitreous film.